Human (HIV) and bovine (BIV) immunodeficiency viruses exploit host cyclin T1 proteins to enhance viral transcription using interactions between cyclin T1, viral Tat proteins, and the TAR RNA hairpins located on viral transcripts. The two viruses have evolved distinct mechanisms for cyclin T1 recognition; however, these differences are not well understood in molecular detail. Biochemical and biophysical experiments will be used to characterize the domains of cyclin T1 that contain both the canonical cyclin fold and the binding region for the Tat:TAR complex. This includes purification of the human and bovine cyclin domains and assessment of protein folding by both spectroscopic assays, and proteolytic mapping. Additionally, individual contributions of cyclin, Tat, and TAR molecules to the formation of the protein-RNA complexes will be assayed through a series of titration experiments and proteolytic mapping. Finally, optimizing the conditions to generate correctly folded complexes will facilitate attempts at growing crystals of the complexes. The molecular details uncovered by these studies are expected to provide new information about transcription control mechanisms and may aid in the search for new therapies to treat viral disease and cyclin related cancers.